Brightness

The luminosity of a star is the total power it radiates in the form of light and radiation in all directions, expressed in watts or as a multiple of the solar luminosity. It is a fundamental and intrinsic property of the star— it does not depend on the distance from which it is observed, unlike apparent magnitude, which decreases with distance.

What does luminosity depend on?

A star’s luminosity depends mainly on two factors: its surface temperature and its radius. The Stefan-Boltzmann law states that luminosity is proportional to the square of the radius and the fourth power of the temperature. A star twice as hot as the Sun, with the same radius, will be 16 times more luminous. A star of the same temperature but twice as large will be 4 times more luminous.

The mass-luminosity relationship

For main-sequence stars, there is a close relationship between mass and luminosity: more massive stars are much more luminous. This relationship is not linear—doubling a star’s mass increases its luminosity by a factor of about 8 to 10. This is why massive stars, despite their abundance of fuel, burn out much faster than low-mass stars.

Importance in Astronomy

Luminosity is used to calculate distances, date star clusters, and understand stellar evolution. Objects of known luminosity—standard candles—allow us to measure cosmic distances by comparing their intrinsic luminosity to their apparent luminosity. Cepheids and Type Ia supernovae are the most commonly used standard candles.

A concrete example

The blue supergiant Rigel, in the constellation Orion, has a luminosity about 120,000 times that of the Sun. Despite being 860 light-years away, it is the seventh brightest star in the sky. If it were the same distance away as Sirius (8.6 light-years), it would shine as brightly as a crescent moon.

Frequently Asked Questions

Are luminosity and magnitude the same thing?

No. Luminosity is an absolute physical measure of radiant power, in watts. Magnitude is a logarithmic scale of brightness, which can be apparent (observed) or absolute (normalized to 10 parsecs). Absolute magnitude is directly related to luminosity.

Is the Sun a bright star?

Compared to the average stars in the galaxy, the Sun ranks in the upper half. Most stars are red dwarfs that are much less luminous. But there are stars that are several orders of magnitude more luminous than it.

Does a star’s luminosity vary over the course of its life?

Yes. The Sun is about 30% brighter today than it was at its birth 4.6 billion years ago, and will continue to increase in brightness. Pulsating variable stars, such as Cepheids, have luminosities that vary periodically over the course of days or weeks.